Well most factions have a certain diversity in units.
Carthage has the most units available behind the Romans, and some different ones too. Egypt, Pontus, Seleucids, all have a good selection and can pursue different strategies (you can make a hit and run army, a solid infantry army, or anything else you'd like).
The ones to watch out for are Gaul, Scythia, and maybe some others like Thrace and Dacia. They have a limited troop selection.

Anyway, I suggest you take a look at the guide I made (link in my sig). You can easily overview all the factions in the game and decide which faction you want. Or you can load up custom battles and try all armies out first to see which you like...